Pregnant 17 weeks. Placenta is on the lower side and baby has big bone structure. Little muscle/fat. Normal?

So i just went for my ultrasound scan and a 3d image, I AM HAVING A BABY GIRL, 90% it would be unless there's a mistake. But, I am more concerned about my low laying placenta. The gynae told me that i was carry too many heavy stuff and walking too much and advises me to rest more this month. And also, my baby is 15cm at this time and half a pound. Is it normal? And from the 3d and what the doctor said, my baby has little meat and fats. Is it normal also?